1. Field of the Invention
The present invention relates to a photographing method and apparatus which photographs an object while performing auto focus of a contrast system.
2. Description Related to the Prior Art
A photographing apparatus, such as a digital camera, includes a focus lens, a lens drive unit which moves the focus lens in an optical axis direction, and an imaging unit which images the object through the focus lens to generate image data. Such a photographing apparatus is normally provided with an auto focus (AF) function of automatically executing a focusing operation without any user operation. As the AF function, a contrast system in which a focal position where image contrast is the highest is detected while moving a focus lens is generally used.
The AF function of the contrast system is realized by a frequency component extraction unit, an AF evaluation value acquisition unit, and a focal position detection unit. The frequency component extraction unit extracts a predetermined spatial frequency component from each piece of image data, which is generated through imaging with an imaging unit at an interval of a predetermined time while moving the focus lens, using a high pass filter. The AF evaluation value acquisition unit integrates the extracted spatial frequency component to acquire an AF evaluation value representing contrast. The focal position detection unit detects the focal position of the focus lens searches by searching for a peak position of the AF evaluation value.
Since the peak of the AF evaluation value does not appear if the focus lens does not actually pass the focal position, when the moving speed of the focus lens is high, the moving pitch of the focus lens is great, and the focus lens may greatly depart from the focal position. In this case, as described in United States Patent Application Publication No. 2011/0261251 (corresponding to WO2010/050200A), after the focus lens passes through the focal position, the moving speed of the focus lens is changed to a low speed to reduce the moving pitch, whereby it is possible to detect the focal position with excellent accuracy. However, it takes a long time until the focus lens is returned to the focal position after greatly departing from the focal position.
In response to this problem, JP2012-103708A suggests that first and second AF evaluation values are acquired using a first high pass filter having a first cutoff frequency and a second high pass filter having a second cutoff frequency higher than the first cutoff frequency, and moving control of a focus lens is performed with the quotient obtained by dividing the first AF evaluation value by the second AF evaluation value as a parameter.
In this parameter, a peak appears at a position separated by a predetermined amount from the focal position. “Coarse adjustment” of the focus lens is performed from when the focus lens starts to move until the peak of the parameter is detected, and “fine adjustment” is performed after the peak is detected. Specifically, moving of the focus lens is temporarily stopped when the peak of the parameter is detected, and thereafter, moving of the focus lens is performed based on the second AF evaluation value. The first and second cutoff frequencies are set such that the peak position of the parameter becomes about twice the moving pitch of the focus lens from the focal position, whereby the time necessary for fine adjustment is reduced and the AF operation time is reduced.
However, in the photographing apparatus described in JP2012-103708A, while coarse adjustment is performed if moving of the focus lens is started and is changed to fine adjustment when the focus lens reaches a position separated by a predetermined amount from the focal position, at this time, control for the moving speed of the focus lens is not performed. For this reason, when the moving pitch of the focus lens is great, since the focus lens greatly departs from the focal position, the AF operation time is extended.
Furthermore, since the distance between the peak position of the parameter and the focal position depends on the first and second cutoff frequencies, in order to reduce the AF operation time, it is necessary to appropriately set the first and second cutoff frequencies. When this setting is not appropriate, the AF operation time is not reduced.